This invention relates to optical devices, and in particular to devices which divert light among various transmission paths.
With the present and potential growth of lightwave communications systems, needs have arisen for a variety of optical components capable of diverting or splitting light signal onto different transmission paths. For example, where the signal includes more than one wavelength, a device capable of multiplexing and demultiplexing the constituent wavelengths is needed. Some proposals have been made to provide this function by a combination of focusing and reflecting elements. For example, U.S. Pat. No. 4,213,677 issued to Sugimoto et al teaches use of two graded index of refraction lenses with a filter element sandwiched between them. When light including two wavelengths is incident from a fiber on one surface of a lens, one wavelength will be reflected by the filter to another fiber on the same surface and the other wavelength will be transmitted to a fiber positioned on the opposite surface of the other lens. An array of fibers can be positioned on each surface to provide multiplexing/demultiplexing of several different signals comprising two wavelengths. U.S. Pat. No. 4,304,460 issued to Tanaka shows a focusing rod lens with an array of optical waveguides on one surface and a tilted, rotatable reflection means near the opposite surface. Between the lens and reflection means is a tilted, rotatable filter which reflects a first wavelength and transmits a second wavelength. By rotating the filter and reflection means separately to a desired location, the two wavelengths from a single waveguide can be made incident on two different waveguides of the array. As another example, the patent suggests that an additional focusing lens be placed behind the filter so that while one wavelength is reflected back to a waveguide, the second wavelength can be transmitted to a transducer which controls the rotation of the filter.
The above-described references are primarily concerned with signals including only two wavelengths. However, in order to increase the capacity of future systems, three or more wavelengths will be desirable. This need creates special requirements for optical components in minimizing losses and properly aligning the various wavelength components with their appropriate fibers. One proposal for a four-channel multiplexer/demultiplexer is found in U.S. patent application Ser. No. 246,106, filed Mar. 20, 1981, now U.S. Pat. No. 4,424,474, and assigned to the present assignee. There, a plurality of filters is provided at one end of a graded index of refraction lens and placed at various angles so that each wavelength component will be reflected by a different filter to a different fiber in an array positioned at the other end of the lens. It is suggested that the angular disposition of the filters can be achieved by means of transparent wedges cemented between the filters. A device of this type has definite advantages, but also suffers from the fact that a significant loss may result each time a constituent wavelength traverses one of the filters. Thus, a constituent wavelength reflected by one of the back filters may become too attenuated to provide a good signal. Further, in the above proposals where filter elements are positioned at an angle to each other, it appears that the angle must be preset within tight tolerances. It may be more advantageous if the angles could be adjusted during manufacture to achieve maximum alignment of the light with its appropriate transmission fibers.
It is also desirable to divert light to different transmission paths when the signal is composed of one wavelength. For example, in data communication systems, often a single central processing unit will service several terminals. Each such terminal should have the capability of extracting a portion of the signal while the remainder of the signal is available to other terminals. Also, each terminal should be able to transmit a signal onto the same line without interfering with its own receiver. Thus, what is needed at each terminal is an asymmetric four-port optical coupler where at least three ports could be operational at one time while the local transmitter and receiver ports are optically isolated. U.S. Pat. No. 4,213,677, cited above, describes a four-port coupler employing a pair of focusing elements with a partially reflecting element sandwiched therebetween and two fibers located on each opposing surface of the focusing elements. Incoming light from one of the fibers will be partially reflected back to the other fiber on the same surface and partially transmitted to one of the fibers at the other surface. The coupler is therefore symmetrical, and any light from a local transmitting port will also be incident on a receiver port.
It is therefore desirable to provide an optical device for diverting light among different transmission paths which in one form is capable of multiplexing and demultiplexing a signal of three or more wavelengths with high efficiency. It is also desirable in such devices to include some means of monitoring the transmission of each constituent wavelength. It is further desirable to provide an optical device for diverting light among different transmission paths which in another form is capable of coupling light among at least three ports simultaneously and providing optical isolation between one of these ports and an additional port to produce an asymmetric four-port coupler.
It is further desirable to manufacture such devices to permit maximum coupling efficiency between the transmission elements for each device.